I would reject your implication that an American disgusted by the My Lai Massacre (or “incident” if you prefer) should be considered anti-American, although lots of Americans did think that.

To answer your question, yes, almost all imperial states have had people who still considered themselves Americans or English or Japanese or whatever but were either deeply ambivalent or outright opposed to “their own” empire. Three examples would be Mark Twain, George Orwell and the Japanese Pan-Asianists.

Twain was disgusted by the Spanish-American War and above all the American conquest of the Philippines. He saw this as a betrayal of America’s fundamental principles, which were to him implicitly anti-imperialist. He thought that Empire was morally wrong, and would lead to all sorts of bad things at home, like a standing army (also un-American, to him and lots of other people at the time.) A lot of this was in his 1901 essay “To the Person Sitting in Darkness”, but you can see his anti-imperialism earlier in things like the 1889 book A Connecticut Yankee in King Arthur’s Court (If you have not read it, it is the story of an American who travels to a primitive place and tries to introduce modern technology, but the people are too backward and he is forced to kill them all.) You can certainly call him Anti-American, and many people at the time did, but he did not see it that way and he was hardly unique.

Orwell is probably the best-known example. “Shooting an Elephant” is probably one of the most widely anthologized essays in the English-speaking world, and it is a direct condemnation of the Empire. He was highly critical of the Empire in almost all his work, but also a patriotic Englishman, at least in his own mind. (Again, you are free to disagree with this, and many people did.) He was fairly typical of the entire European Socialist movement in this, in that they were often anti-imperialist. Orwell was fairly typical in pointing out that the Empire benefitted the “Big Capitalists”, but all ordinary people saw out of it was higher taxes and a chance to get shot. Organizations like the Empire League were set up to encourage pro-Empire feeling among ordinary people in Britain, but Orwell claimed that they remained mostly indifferent. He completely rejected the idea the England and the Empire were one and the same. (He almost always called himself English rather than British.)

The Japanese Pan-Asianists are slightly different. Pan-Asianism eventually became a slogan to cover up Japanese Imperialism, but many of the people on the ground running railways in China’s Northeast or schools in Taiwan legitimately saw themselves as helping fellow Asians. Lots of Chinese and Koreans went to school in Japan and had teachers and friends who encouraged them (Lu Xun is an example). These people often became quite unhappy with what the Japanese Empire became, although there was not much they could (or would) do about it.

All of these people were complicit in Empire in some ways. Many of the Japanese had deeply condescending (Orientalist, really), ideas about their little Asian brothers. Twain would not have put the American conquest of the Great Plains in the same category as the Philippines. They were often aware of this contradiction, Orwell most of all. “Inside the Whale” is one of his other well-known essays, and it points out that one of the reasons England has so many left-wing writers like him is that it has a huge empire that makes it a very rich country.

There are lots of other examples. Romans who thought that Empire was corrupting the virtues of the Old Republic, lots of Christian missionaries who were ambivalent or outright hostile to their home countries activities. A lot of the work on U.S. power around the world is written by former Peace Corps people.

So, in answer to your question, the U.S. is in no way unusual in having people who question their “own” empire.

Some sources Atkins, E. Taylor. Primitive Selves: Koreana in the Japanese Colonial Gaze, 1910–1945. Berkeley: University of California Press, 2010.

Lai-Henderson, Selina. Mark Twain in China. Stanford, California: Stanford University Press, 2015.

Rodden, John. The Politics of Literary Reputation: The Making and Claiming of “St. George” Orwell. 2nd Printing edition. New York: Oxford University Press, 1989.

Saaler, Sven, and J. Victor Koschmann, eds. Pan-Asianism in Modern Japanese History: Colonialism, Regionalism and Borders. New York. Routledge, 2006.

Saaler, Sven, and Christopher W. A. Szpilman, eds. Pan Asianism: A Documentary History, Vol. 2, 1920-–Present. Lanham, Md: Rowman & Littlefield Publishers, 2011.